[Lalwendë]

there in Avathar, secret and unknown, Un- goliant had made her abode. The Eldar knew not whence she came; but some have said that in ages long before she descended from the darkness that lies about Arda, when Melkor first looked down in envy upon the Kingdom of Manwe, and that in the beginning she was one of those that he corrupted to his service. But she had disowned her Master, desiring to be mistress of her own lust, taking all things to herself to feed her emptiness; and she fled to the south, escaping the assaults of the Valar and the hunters of Orome, for their vigilance had ever been to the north, and the south was long unheeded. Thence she had crept towards the light of the Blessed Realm; for she hungered for light and hated it.

There's a crucial point - that she came to the world even before Melkor, and 'out of the darkness'. She certainly wasn't a creature created by him, and this also suggests she wasn't one of the Ainur either or what was she doing there before the others?

There is also a very strong argument that metaphorically she could be an equal and opposite or even a necessary complement to Eru. He is symbolised by Light and in Arda Light is symbolic of Life. Ungoliant on the other hand symbolises more than mere darkness, she is Un-light. Read the following:

Yet no song or tale could contain all the grief and terror that then befell. The Light failed; but the Darkness that followed was more than loss of light. In that hour was made a Darkness that seemed not lack but a thing with being of its own: for it was indeed made by malice out of Light, and it had power to pierce the eye, and to enter heart and mind, and strangle the very will.

What Ungoliant does is not merely consume the Light but replace it with Unlight. She achieves something which Melkor simply could not do, even with his power. He has to enlist her to do this for him, and she does, but then she deserts him once again to be 'mistress of her own lusts'. Doesn't this suggest Ungoliant has a terrible power even Melkor did not possess?

Now, something for The Might...more on Moru:

Originally Posted by BoLT

for here dwelt the primeval spirit Moru who even the Valar know not whence or when she came, and the folk of Earth have given her many names. Mayhap she was bred of mists and darkness on the confines of the Shadowy Seas, in the utter dark that came between the overthrow of the Lamps and the kindling of the Trees, but more like she has always been;

Originally Posted by BoLT Commentary

In the tale her origin is unknown, and though this element may be said to have remained in The Silmarillion..., by the device of 'Some have said...' a clear explanation is in fact given: she was a being from 'before the world', perverted by Melkor...The original idea of 'the primeval spirit Moru' is made explicit in an entry in the early word list of the Gnomish language, where the name Moru is defined as 'a name of the Primeval Night personified as Gwerlum or Gungliont'

I suppose the thorny question of whether Ungoliant came about before or after Eru created the Ainur could be answered by considering that which you decide depends upon whether you are content that Eru could create Ungoliant? Or does it 'suit' that maybe she comes from outside Eru's creation?